U.S. patent application number 11/038704 was filed with the patent office on 2005-08-11 for method for continuous cooking.
Invention is credited to Gustavsson, Lennart, Saetherasen, Jonas, Snekkenes, Vidar.
Application Number | 20050173081 11/038704 |
Document ID | / |
Family ID | 31885252 |
Filed Date | 2005-08-11 |
United States Patent
Application |
20050173081 |
Kind Code |
A1 |
Snekkenes, Vidar ; et
al. |
August 11, 2005 |
Method for continuous cooking
Abstract
A procedure for the continuous cooking of chemical pulp,
comprising an impregnation vessel 101 and a digester 102. The chips
are impregnated in the impregnation vessel 101 and are fed after
impregnation together with circulation fluid through a transfer
line 103 to a top separator 105 on the digester 102 in order to be
cooked. A fraction of the circulation fluid is withdrawn at a top
separator 105 and returned to the bottom of the impregnation vessel
101 via a return line 106. Black liquor is withdrawn from the
digester via a black liquor line 108 to the impregnation vessel
101. The invention is characterised in that a heat exchanger 107 is
arranged between the return line 106 and the black liquor line 108.
The heat exchanger 107 allows the exchange of heat between the two
lines, but does not allow the exchange of fluid. The temperature of
the black liquor in the black liquor line 108 after passing through
the heat exchanger 107 has been cooled by 10-15 .degree. C.
Inventors: |
Snekkenes, Vidar; (Karlstad,
SE) ; Gustavsson, Lennart; (Karlstad, SE) ;
Saetherasen, Jonas; (Karlstad, SE) |
Correspondence
Address: |
FASTH LAW OFFICES (ROLF FASTH)
26 PINECREST PLAZA, SUITE 2
SOUTHERN PINES
NC
28387-4301
US
|
Family ID: |
31885252 |
Appl. No.: |
11/038704 |
Filed: |
January 19, 2005 |
Current U.S.
Class: |
162/29 ; 162/41;
162/52; 162/6 |
Current CPC
Class: |
D21C 3/24 20130101; D21C
7/10 20130101; D21C 7/14 20130101; D21C 3/02 20130101 |
Class at
Publication: |
162/029 ;
162/041; 162/006; 162/052 |
International
Class: |
D21C 003/24 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 9, 2004 |
SE |
0400253-1 |
Claims
1. A procedure for the continuous cooking of cellulose pulp,
comprising: providing an impregnation vessel in fluid communication
with a digester, feeding a mixture of chips and process fluid to
the impregnation vessel, impregnating the chips at a predetermined
impregnation temperatures T.sub.imp, feeding the impregnated chips
together with a circulation fluid through a transfer line to a top
separator on the digester, cooking the chips at a predetermined
cooking temperature, T.sub.dig, where T.sub.imp is at least
20.degree. C. lower than T.sub.dig, withdrawing a fraction of the
circulation fluid at the top separator, returning the circulation
fluid to a bottom of the impregnation vessel via a return line,
withdrawing black liquor from the digester at a withdrawal location
to a black liquor line that essentially maintains the cooking
temperature, T.sub.dig, and essentially a full digester pressure
with a first level of residual alkali, providing a heat exchanger
between the return line and the black liquor line for exchanging
heat between the return line and the black liquor line, cooling the
black liquor in the black liquor line by 10-15.degree. C. after
passing through the heat exchanger, withdrawing more than 1
m.sup.2/tone wood and/or at least 25% of a total withdrawal for
recovery (RECtot) from the circulation fluid for recovery (REC1),
heating the circulation fluid in the return line at the heat
exchanger, leading the black liquor in the black liquor line to an
upper section of the impregnation vessel after passing through the
heat exchanger, the black liquor contributing to an establishment
of a f/w ratio that exceeds 4 in the impregnation vessel, obtaining
more than 40% of the f/w ratio in the impregnation vessel from the
black liquor in the black liquor line, providing the circulation
fluid in the return line with a second level of residual alkali,
ensuring that the first level of residual alkali is at least 10 g/l
higher that the second level of residual alkali, and consuming at
least 60 kg NaOH per ton of wood of alkali in the impregnation
vessel.
2. The method according to claim 1 wherein the first level of
residual alkali is greater than 20 g/l.
3. The method according to claim 2 wherein the first level of
residual alkali is withdrawn from the digester after a retention
time in the digester of at least 30 minutes.
4. The method according to claim 2 wherein the second level of
residual alkali lies within an interval of 5-10 g/l.
5. The method according to claim 4 wherein the chips are given a
retention time during the impregnation of at least 40 minutes in
the impregnation vessel.
6. The method according to claim 1 wherein white liquor is added at
a top of the digester, the addition of white liquor increases a
quantity of OH.sup.- by at least 10 g/l and/or is equivalent to at
least 50% of a total addition of white liquor for impregnation and
cooking.
7. The method according to claim 1 wherein a withdrawal of fluid
for recovery (REC2) is carried out from a location in the digester
at which the pulp has had a retention time in the digester that
exceeds a retention time for the withdrawal of black liquor at the
withdrawal location by at least 60 minutes.
Description
TECHNICAL AREA
[0001] The present invention concerns a method for continuous
cooking of cellulose pulp according to the introduction to claim
1.
THE PRIOR ART
[0002] The withdrawal of consumed cooking fluid, known as black
liquor, at the cooking temperature from various positions of the
digester with a certain level of residual alkali during the
continuous cooking of cellulose pulp in a two-vessel system,
subsequently to lead the consumed cooking fluid after full or
partial release of pressure, at a temperature of approximately
90-120.degree. C., to an impregnation vessel for impregnation, is
known, and is termed "black-liquor impregnation". The principal aim
of this type of impregnation is to obtain an improved impregnation
of alkali black liquor such that the chips are totally neutralised
and given an alkali pH, and to achieve a high level of sulphide in
the chips before the cooking, such that a pulp of higher quality is
obtained.
[0003] A further aim is to be able to conserve to a greater degree
the heat in the withdrawn cooking fluid in order to heat the colder
chips in the impregnation vessel.
[0004] The conservation of part of the heat from the hot black
liquor in association with black liquor impregnation is also known.
This traditionally takes place through pressure reduction of steam
using pressure-reduction cyclones, in a process in which this flash
steam is used for, among other purposes, steam-treating the chips,
or for other heating purposes.
[0005] A short black liquor impregnation at the top of the digester
essentially at cooking temperature was used in the older methods of
black liquor impregnation. It was considered to be advantageous to
use a high temperature during the impregnation such that it should
proceed rapidly and efficiently. An impregnation under
countercurrent flow was considered to be particularly advantageous
for a thorough impregnation.
[0006] The trend in recent years has been to impregnate using black
liquor at a lower temperature and with a greater part of the
impregnation under concurrent flow and for a longer period,
typically 60-120 minutes. One of the major advantages of this is
that the amount of xylan released becomes lower with lower
temperatures. Cooling of the withdrawn cooking fluid (the black
liquor) has for this reason been necessary, and this has taken
place either through flashing or through cooling in a heat
exchanger with water as cooling agent.
[0007] However, a lower temperature during the impregnation
involves the need to warm the chips when they proceed from the
impregnation vessel to the digester. This has been solved through
heaters in the transfer flow. Heat exchangers with steam as the
heating agent have been most often used as heaters. This method
requires considerably energy, and for each cubic metre of steam
that is used for this purpose, the possible production of
electrical power from the steam is reduced. It is therefore
desirable to discover methods that allows impregnation at low
temperature in which the heat in the black liquor can be conserved
and passed to the digester without these energy losses arising, or
at least, being reduced to a minimum.
[0008] The Swedish patent SE 518 957 reveals a continuous cooking
process with the aim of improving the heat economy in association
with black liquor impregnation, that wholly or partially removes
the problems and disadvantages described above.
[0009] Black liquor, the pressure of which has not been fully
released, is passed in this patent into the lower zone of the
impregnation vessel in order to increase the temperature of the
chips before the digester. Parts of the black liquor are withdrawn
at the top separator above the digester and returned to the
impregnation zone of the impregnation vessel. In this way, the hot
black liquor participates in increasing the temperature of the
chips before their transfer to the digester, whereby the
requirement for heating at the top of the digester is lowered.
[0010] SE 502 134 reveals a method for optimising the fluid/wood
ratio in impregnation vessels and digesters. The liquor in the
transfer circulation is partially separated in the top separator of
the digester and mixed with an amount of cooking fluid withdrawn
from the digester from a strainer section at a lower level in the
digester. This mixture of liquor is led together through a heat
exchanger for heating back to the outlet arrangement at the bottom
of the impregnation vessel. However, a subsidiary current of this
impregnation fluid is led without being heated to the top of the
impregnation vessel in order for a higher fluid/wood ratio to be
obtained at the top of the impregnation vessel. One of the
advantages is, according to the patent, that the flow of steam for
heating at the top of the digester can be somewhat reduced in that
the temperature is higher in the transfer.
[0011] The disadvantage of this patent is that the levels of
residual alkali in the return liquor from the top separator and
from the strainer section lower down in the digester should not
deviate by too large a degree. If the residual alkali levels in
these two flows deviates by too large a value, a resulting mixture
of return liquor is obtained that has neither a "high" level of
residual alkali nor one that is sufficiently "low" for it to be
sent for recovery. This gives rise to large differences in the
level of residual alkali, something that means that batchwise
addition of alkali and its profile cannot be optimised, and that
there is a risk that the addition of fresh alkali passes directly
to recovery without having had a substantial retention time in the
cooking system.
AIM AND BRIEF DESCRIPTION OF THE INVENTION
[0012] The principal aim of the present invention is to achieve a
continuous cooking process with an improved heat economy, and:
[0013] to impregnate at a lower temperature, for a long time and
where a sufficient quantity of alkali is included in the
impregnation fluid, with the aim of retaining the xylan in the
cellulose;
[0014] that the chips are warmed at the bottom of the impregnation
vessel in order to reduce the requirement for steam at the top of
the digester;
[0015] to achieve a good alkali profile for the impregnation vessel
and the digester;
[0016] that separation of partially consumed cooking fluid and
totally consumed cooking fluid can be achieved, such that only
partially consumed cooking fluid is used for black liquor
impregnation in which the consumption of alkali is high, and;
[0017] that addition of fresh white liquor at the top of the
digester is prevented from being drawn to recovery with only a
short retention time in the system, something that gives a high
efficiency of the batchwise addition of alkali.
[0018] This is achieved with a method according to claim 1.
[0019] A method for the continuous cooking of cellulose in a
two-vessel cooking system is offered through the present invention
where impregnation takes place in an impregnation fluid that at
least partially comprises withdrawn partially consumed cooking
fluid at the cooking temperature from the digester, otherwise known
as "black liquor". The method makes it possible to impregnate at
low temperatures in agreement with the most recent development
within the field of black liquor impregnation, while at the same
time the need for cooling of the black liquor to the impregnation
vessel is reduced or eliminated.
[0020] The method also reduces or eliminates the need for heating
in the transfer line between impregnation vessel and digester,
which indirectly reduces the consumption of pure steam or flash
steam which can, thus, be used at another location, while it
reduces the need of supplementary steam addition at the top of the
digester in order to raise rapidly the temperature of the chips to
the cooking temperature. The method, considered comprehensively,
provides an improved energy economy relative to that which is
previously known in that the energy losses that unavoidably arise
when carrying out heat-exchange with steam, flashing, etc., are
reduced.
[0021] Furthermore, the method provides a good alkali profile not
only for impregnation but also for cooking. This is particularly
true for cooking processes in which the difference in the levels of
residual alkali in the circulation fluid in the return line from
the top separator to the bottom of the impregnation vessel and the
withdrawn cooking fluid (known as black liquor) from the digester
to the upper part of the impregnation vessel is at least 10
g/l.
DESCRIPTION OF DRAWING
[0022] FIG. 1 shows schematically one preferred embodiment of a
two-vessel cooking plant in which the application is applied.
DETAILED DESCRIPTION OF THE INVENTION
[0023] The concept of "black liquor" will be used in the
description below. "Black liquor" is here taken to denote consumed
or partially consumed cooking fluid that is withdrawn from the
digester 102 at a cooking temperature in the interval
130-160.degree. C., typically 140.degree. C., and in which the
level of residual alkali is suitably greater than 20 g/l. However,
in order to be characterised as black liquor, the withdrawal must
take place after cooking for at least 30 minutes, preferably after
45 minutes. One skilled in the arts will realise that the position
of the withdrawal will vary, depending on the particular cooking
method and the cooking conditions associated with it, and this
position can thus comprise a withdrawal at the beginning, in the
middle, or at the end of the digester in a zone of concurrent flow
or of countercurrent flow, or as a withdrawal between an upper zone
of concurrent flow and a subsequent zone of countercurrent flow.
More than one withdrawal may also be used.
[0024] Furthermore, the concepts "recovery RECL", "recovery
REC1.sub.ALT", "recovery REC2" and "recovery RECtot" will be
used.
[0025] "REC1" is here used to denote a portion of the circulation
fluid in the return line 106 from the top separator 105 to the
bottom of the impregnation vessel, and where this portion is
directly forwarded to recovery or is forwarded indirectly to
recovery via impregnation.
[0026] "REC1.sub.ALT" is here used to denote an alternative
embodiment to that of REC1, in which a portion is instead withdrawn
from the impregnation vessel.
[0027] "REC2" is here used to denote a withdrawal of fluid that is
withdrawn from the digester 102 and where this fluid is forwarded
to recovery.
[0028] "RECtot" is here used to denote the total quantity of all
fluids that are forwarded from the digester 102 for recovery or for
impregnation.
[0029] Finally, the concepts "first level of residual alkali" and
"second level of residual alkali" will be mentioned. "First level
of residual alkali" is here used to denote the level of residual
alkali of the withdrawn partially consumed cooking fluid, known as
black liquor", that is carried out at a withdrawal position 109
through a withdrawal strainer 114 into the black liquor line 108.
"Second level of residual alkali" is here used to denote the level
of residual alkali in the withdrawal of circulation fluid that is
carried out via the top separator 105 into the return line 106.
[0030] FIG. 1 shows schematically a method for a continuous
two-vessel digestion plant for the production of cellulose pulp in
which the invention is applied and where the cooking system
comprises an impregnation vessel 101 and a digester 102. The
digester 102 is of steam/fluid phase type, with a top separator 105
at the top, but the invention can also be applied at a hydraulic
cooking system with separation of the chips and the transport fluid
in a strainer section at the top of the digester.
[0031] The impregnation vessel 101 has an inlet at which chips
together with process fluid (chip moisture, any condensate from a
preceding steaming if this is used, and a certain addition of
alkali for the purposes of adjustment) are fed into the vessel 101
for impregnation of the chips at a predetermined impregnation
temperature, T.sub.imp, that lies in the interval 100-140.degree.
C.
[0032] The consumption of alkali in the impregnation vessel 101 is
at least 60 kg NaOH per tonne of wood, and this requires a
retention time of at least 40 minutes and typically in the interval
40-120 minutes. The greater part of the alkali is consumed in
neutralising the naturally acidic chips and in establishing an
alkali pH for the complete chip section. A minor fraction of the
delignification also takes place during the impregnation, but only
a negligible fraction of the bulk delignification.
[0033] The impregnated chips are fed together with circulation
fluid into a transfer line 103 to the top separator 105 on the
digester 102. The chips are cooked in the digester 102 at a
predetermined temperature, T.sub.dig, that lies in the interval
130-180.degree. C.
[0034] In all conditions according to the invention T.sub.imp is at
least 20-30.degree. C. lower than T.sub.dig.
[0035] Once the impregnated chips have been transported in the
transport line 103 from the impregnation vessel 101 to the top
separator 105, a portion of the circulation fluid is withdrawn at
the top separator 105 and this portion is returned to the bottom of
the impregnation vessel via a return line 106. This withdrawal of
circulation fluid in the return line 106 has a second level of
residual alkali. This second level of residual alkali lies in the
interval 5-10 g/l and is directly suitable for sending directly to
recovery.
[0036] At the top of the digester (following, or in association
with, the outlet from the top separator) the quantity of white
liquor that is required for the bulk delignification phase is
subsequently added. Typically, a quantity of fresh alkali (white
liquor) is added such that the level of OH.sup.- increases by at
least 10 g/l and/or equivalent to at least 50% of the total
addition of fresh white liquor for impregnation and digestion. The
total batch of alkali (both alkali from the fresh white liquor and
residual alkali in the accompanying black liquor) at the top of the
digester is greater than 100-120 kg NaOH per tonne of wood. At
least one withdrawal of partially consumed cooking fluid, known as
black liquor, is carried out from the digester 102 at at least one
withdrawal strainer 114 at withdrawal location 109 to a black
liquor line 108, and this withdrawal is led to the upper section of
the impregnation vessel 101. The black liquor from the black liquor
line 108 contributes to a f/w ratio (a fluid/wood ratio) that
exceeds 4, preferably one that exceeds 5, being established in the
impregnation vessel 101. This withdrawal of black liquor
essentially maintains at the withdrawal location 109 the cooking
temperature, T.sub.dig, and full cooking pressure, and it contains
a first level of residual alkali. It is appropriate that this first
level of residual alkali is higher than 20 g/l. This black liquor
with a first level of residual alkali is withdrawn from the
digester after a retention time for the pulp in the digester of at
least 30 minutes, preferably longer than 45 minutes. More than 40%
of the f/w ratio in the impregnation vessel 101 is obtained from
this black liquor in the black liquor line 108. For all cooking
conditions according to the invention, the first level of residual
alkali in the hot black liquor in the black liquor line 108 after
its withdrawal is at least 10 g/l higher than the second level of
residual alkali in the return line 106 of the circulation
fluid.
[0037] A heat exchanger 107 is arranged between the return line 106
and the black liquor line 108 that allows heat exchange, without
allowing exchange of fluid, between the return line 106 and the
black liquor line 108. After passage of the black liquor through
the heat exchanger 107, the temperature of the black liquor in the
black liquor line 108 has been reduced by 10-15.degree. C. The
temperature of the circulation fluid in the return line 106 has
been raised by an amount that is equivalent to the amount of energy
that has been released by the cooling of the black liquor in the
black liquor line 108.
[0038] The temperature difference in the return line 106 is small
due to the flow from the circulation fluid in the return line 106
in the heat exchanger 107 being very much greater than the flow of
black liquor in the black liquor line 108. The flow in the return
line is typically at least 3-5 times greater than the flow in the
flow of the chips mixture through the impregnation vessel. The
withdrawal flow of black liquor in the black liquor line 108 is, in
turn, often less than 50% of the flow in the chips mixture down
through the impregnation vessel 101. On the other hand, the rise in
temperature over the system, that is, the rise in the temperature
of the chips from the impregnation vessel to the top of the
digester, is greater when measured in terms of the number of
Celsius degrees through which the temperature of the chips is
raised.
[0039] Before the circulation fluid in the return line 106 is
heated in the heat exchanger 107, more than 1 m.sup.3/tonne wood is
withdrawn at the recovery REC1, and/or at least 25% of the total
quantity that is sent to recovery, RECtot.
[0040] It may be possible to replace this withdrawal location by a
withdrawal strainer 110 at the bottom of the impregnation vessel,
which is, actually, a conventional withdrawal location in older
two-vessel systems.
[0041] At a location in the digester 102 at which the pulp has had
a retention time in the digester that significantly exceeds the
retention time for the withdrawal of black liquor at the withdrawal
location 109, by at least 60 minutes, it is appropriate that a
withdrawal of fluid is carried out for recovery REC2 via a
withdrawal strainer 115.
[0042] Further fluid circulations may, possibly, be arranged within
the digester with heating or with adjustment of the cooking fluid,
where certain of these may involve partial withdrawal of consumed
cooking fluid.
[0043] The use of the heat exchanger 107 to heat the circulation
fluid in the return line 106 by direct heating and simultaneously
cool the black liquor in the black liquor line 108 allows the
following positive effects to be achieved:
[0044] An improved heat economy at the given process conditions
(time and temperature in the various zones).
[0045] The chips at the bottom of the impregnation vessel are
heated before the transfer line 103, as a result of which the
quantity of steam required to heat the chips at the top of the
digester can be significantly reduced.
[0046] The temperature is held at a low value during the main part
of the impregnation, something that is advantageous for the
impregnation and that reduces the quantity of xylan released.
[0047] The alkali levels of the circulation fluid in the return
line 106 and of the black liquor in the black liquor line 108 are
unchanged following the exchange of heat between these two, where
the one fluid has a higher alkali level and a reduced temperature
suitable for the impregnation, and the second fluid has a lower
alkali level and a higher temperature, with the principal aim of
heating the chips during the shorter retention time in the transfer
system.
[0048] The invention is not limited to the embodiments described,
and several variants are possible within the scope of the
claims.
[0049] For example, a second cooler can be located arranged after
the heat exchanger if it is required to reduce the temperature of
the black liquor further. It is appropriate that a fraction of the
cooled black liquor can be added at the inlet to the impregnation
vessel, as is suggested in FIG. 1 by the dashed flow 112. The
second cooler 113 can be installed instead in this flow.
[0050] The withdrawal strainer 114 can also be in the form of
several withdrawal strainers placed at different heights in the
digester, where the level of residual alkali of the mixture
obtained by the withdrawal flow from these strainers can form the
required "high" level of residual alkali.
* * * * *